British prime minister David Cameron has argued his case for the UK to join air strikes in Syria ahead of a vote expected at a later date, with signs of opposition weakening after the Paris attacks.

"If we won't act now, when our friend and ally France has been struck in this way, then our friends and allies can be forgiven for asking: If not now, when?" Mr Cameron asked parliament.

Mr Cameron argued there was a legal basis for intervention for self-defence because of the threat posed by Islamic State jihadists at home, and said Britain should not "sub-contract" its security to allies.

"We have to deny a safe haven for ISIL in Syria. The longer ISIL is allowed to grow in Syria, the greater the threat it will pose," he said in a written statement on the issue, using another acronym for IS.

Mr Cameron called for "patience and persistence" and outlined a seven-point strategy for Syria, including diplomatic and humanitarian efforts and planning for what will happen if president Bashar al-Assad falls.

Mr Cameron is expected to call a vote in parliament on the issue before recess begins on December 17.

This would come two years after a previous vote for military action in Syria failed after the main opposition Labour Party voted against.

"The events in Paris have clearly changed things," Malcolm Chalmers, research director at the Royal United Services Institute (RUSI), told AFP.

"I think the mood in parliament has changed," he said, predicting that the vote will pass since "a significant number of MPs" had changed their minds.

"There's scepticism on both sides of the Houses but I think opinions are beginning to change," he said.

But Mr Chalmers also said there was still a "shadow" from Britain's participation in the US-led invasion of Iraq in 2003 and the NATO bombing campaign in Libya in 2011 which helped topple dictator Moamar Gaddafi but was followed by bitter civil war.

Joining Syrian air strikes could pose a risk to Britain

Critics have argued that joining the campaign could increase the risk of Britain becoming a target.

"As long as we intervene in the Middle East, we must expect atrocities in return. Bombing will not stop them," columnist Simon Jenkins wrote in the Evening Standard this week.

But Mr Cameron on Thursday argued that Britain was already a target, pointing to the killings of 30 British tourists by an IS gunman in a Tunisian resort in June in which a total of 38 people were killed.

He also said that Britain was already assisting in the air campaign on Syria with surveillance.

While British forces are taking part in air strikes on IS targets in Iraq, they are not involved in the US-led coalition targeting Syria due to resistance from opposition parties still mindful of previous unpopular interventions in Iraq and Afghanistan.

Labour's anti-war leader Jeremy Corbyn is against any military action but Mr Cameron appears increasingly confident he can get enough support from Labour MPs to pass the vote, particularly after last week's UN Security Council resolution authorising countries to "take all necessary measures" against IS.

A Times/YouGov opinion poll last week found that 58 per cent of people would approve of Britain joining air strikes in Syria, compared to 22 per cent against.

Reports suggest the government could call a vote on the issue next week.

On Monday, Mr Cameron said the vote could come "in the coming days and weeks".